Because it's a constant.
ok flameduck, you'll have to explain more because it seems like it could be both, like Pi.
Well there are five blend modes. How is BlitzMax going to know which one you want to use unless you tell it? Only one of them can be the default. The other four, you're going to have to tell the compiler what you're doing.
Well if alpha<1, blitz could just use its alphablending mode, unless theres some conflict or some situation when the programmer wouldn't want it on alphablend when he set alpha to <1???